Thermistor bolometers utilizing metal backing blocks



1 E. M.-WORMSER ETAL 2,963,673

THERMISTOR BOLOMETERS UTILIZING METAL BACKING BLOCKS Filed Aug. 16, 1955 Eric fi fi%? 5ier Russgl .27. De Wizard IOOO ATTORNEY I I0 I00 GHOPPING FBEQUENOY IN GYOL United States Patent THERMISTOR BOLOMETERS UTILIZING METAL BACKING BLOCKS Eric M. Wormser, Stamford, and Russell D. De Waard, Old Greenwich, Conn.. assignors to Barnes Engineering Company, Stamford, Conn.

Filed Aug. 16, 1955, Ser- No. 528,797

6 Claims. (Cl. 338-18) This invention relates to an improved construction for radiation sensitive devices. More specifically it relates to improvements in the construction of infra-red sensitive thermistor bolometers to permit tailoring of their characteristics for particular applications, and the manufacture of thermistor bolometers having greater responsivity than previous devices of this kind while maintaining fast response rates.

Thermistor bolometers are useful for measuring infrared radiation, and thus can be used for all types of temperature measurements. In some applications they should have fast response rates to detect rapid changes in temperature or pulses of infra-red energy. In other uses it is desirable that these detectors have a very high responsivity, which is defined as the ratio of the signal developed by the bolometer to the energy falling upon it. In some applications a bolometer having characteristics which represent a compromise between these two extremes is desirable.

Prior to this invention, thermistor bolometers consisted of a thin flake of resistance material with a high negative temperature coeflicient cemented to a backing block of homogeneous heat conducting material. The flake and block assembly were mounted in a metallic housing, having a window transparent to infra-red radiation. Changes in radiation falling on the flake through the window caused it to vary in temperature, thus changing its resistance. This change was detected by applying a direct voltage to the flake and measuring the change in current flowing in this biasing circuit. Since the flake was thus electrically polarized, it was necessary to insulate it from any electrical conductor. In previous devices flake insulation has been accomplished by using a backing block of electrical insulating material since the adhesive layer between the flake and the block is usually too thin to provide effective insulation. The backing block also serves as a thermal conductor to carry away heat generated in the flake due to biasing current and radiation; it is thus termed a thermal sink.

The thermal conductivity of the backing block determines in part the speed of response and the responsivity of the thermistor bolometer. To obtain fast response rates it is desirable that the backing block conduct the heat away from the flake very rapidly. However, if high responsivity is desired the backing block should conduct heat away relatively slowly since the greater the temperature change in the flake, the greater will be its resistance change and therefore the larger will be the developed signals.

The use of electrical insulating materials with high thermal conductivity as backing blocks has been favored in prior devices because such materials give fast response rates and permit the use of relatively high bias voltages since the heat developed by the biasing current is carried away rapidly. Because the electrical signal depends on both the resistance change and the bias voltage, the increase in bias voltage compensates for the decrease in responsivity caused by the high thermal conductivity of Y the backing block. Thus prior devices used backing blocks of material such as quartz, beryllium oxide, magnesium oxide or sapphire to achieve fast response rates. Although these materials gave high speeds, their responsivity was relatively low. If their speeds were reduced by thickening the cement layers, better responsivity could be obtained by slowing the response rate. However both high speed and high responsivity were not generally available in the same device. Some attempts have been made to use metal blocks coated with layers of glass, quartz or mica but such backing blocks produced no better results than solid blocks of the coating material.

Another major problem of prior devices was to control the response rate in manufacture. For certain purposes it is desirable that the bolometers have a slower rate of response than the fastest which can be obtained, with a corresponding increase in responsivity. However, the response rate is determined in devices using the construction described above in part by the material of the backing block, and in part by the material and thickness of the cement layer. Thus to control the time constant it was necessary to control accurately the thickness and uniformity of the cement layer, a difiicult production problem.

Accordingly, it is an object of'this invention to provide an improved construction for thermistor bolometers which will result in better signal to noise ratios than those heretofore obtained. Another object of this invention is to provide a construction of the above character which results in greater responsivity. Still another object of this invention is to provide a construction of the above character which will permit accurate determination of the speed of response and the responsivity during manufacture. Another object is to provide a construction of the above character which is simple and economical in construction. A still further object is to provide a construction of the above character in which the high response rates of previous devices are maintained. A final object is to provide a construction of the above character which will maintain its properties over wide temperature ranges. Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

- For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

Figure 1 is a top elevation of our improved thermistor bolometer having the features of this invention incorporated therein,

Figure 2 is a vertical sectional view of our thermistor bolometer taken along the line 22 of Figure 1, except for the lead details which are shown in elevation for greater clarity,

Figure 3 is an enlarged sectional diagrammatic view of the radiation sensitive portion of our thermistor bolometer and is taken roughly along the line 3--3 of Figure 1, but certain parts shown in Figure l are eliminated.

Figure 4 is a chart showing the relative responsivities of thermistor bolometers having various types of backing blocks plotted as a function of frequency.

Generally speaking, we have discovered that metal backing blocks having a thin plastic film cemented thereto will give greatly improved performance in thermistor bolometers of the type herein described. The thermally sensitive resistor flake is cemented to the film, and is thereby electrically insulated from the metal backing block. The film must therefore be an electrical insulator, and because of its extreme thinness it must have a high dielectric strength to withstand the voltage gradient between the electrically excited flake and the grounded metal backing block. Although the backing block is preferably of metal, any material having comparable thermal conductivity would be satisfactory for use in our invention. As mentioned above, previous attempts to use coatings of electrical insulators on metal backing blocks have resulted in thermistor bolometers with response rates similar to those obtainable from the homogeneous backing materials which are electrical insulators. According to our present understanding, two conditions caused the relatively poor performance of previous coated metal backing blocks. Usually the insulating films were quite thick and were non-uniform while those contemplated in this invention are thin and of uniform thickness. The plastic films which are used herein are less than 50 microns in thickness some being as thin as 6 microns and uniform to a small percentage of their thickness. A film of 50 microns thickness is approximately /3 as thick as the average human hair and 6 microns is that much smaller. A second condition which caused slow response in prior devices was the use of curled or non-flat flakes of thermistor material. As described in the co-pending application of the applicant, Eric M. Wormser, Serial No. 459,017, filed September 29, 1954, entitled Improved Radiation Sensitive Devices, the use of non-flat flakes results in cement layers between the flake and the backing block which are non-uniform and of average thickness much greater than the thinnest portion. These non-uniform films result in increased thermal resistance between the flake and the backing block and slower response rates. For these reasons prior constructions using coated backing blocks have not achieved fast response rates.

An important advantage of using plastic coated metal backing blocks made according to our invention is the increased responsitivity thereby obtained. Since almost the entire thermal resistance of the backing material is concentrated between the flake and the thermal sink in a thin plastic layer in close proximity to the flake, the heat energy not dissipated in the flake is dissipated almost entirely in the plastic film, which in turn heats the flake to a higher temperature than would otherwise be obtained. Thus although the total thermal resistance between the flake and the further side of the sink is approximately the same with backing blocks of this invention and those of quartz or beryllium oxide, the concentration of this resistance close to the flake results in higher responsivity, Without sacrifice of response speed.

As previously mentioned, we propose to use plastic films less than 50 microns in thickness for the plastic films on the backing block providing electrical insulation between block and flake. Films of controlled, uniform thickness are available in this range in the plastic materials which we prefer for use in our invention. Since almost the entire thermal resistance of the backing block is concentrated in the plastic film by selecting a film of the desired thickness, the time constant of the thermistor bolometers made in such manner can be set in manufacture, and thus tailored to a particular application.

Referring to the drawings in detail and particularly to Figures 1 and 2 we have here shown a construction which may be used to house and support our improved thermally sensitive device, it being understood that other structures could be utilized for this purpose. As shown herein a housing for a thermistor bolometer is generally indicated at 10, comprising a flat base 12, secured to a cylinder 14 to form a housing for the other parts of the device. Cylinder 14 has annular bores 16 and 18 adjacent its ends, base 12 resting in bore 18 and being held in place by a solder seal or fillet 20. This base 12 is preferably copper, or a steel alloy which has the same coeificient of expansion as certain glasses and thus can be attached directly to them without resulting heat damage. A window 24 which is transparent to infrared energy and hence preferably made from thallium bromide iodide, a synthetic optical crystal, or silver chloride, is cemented or otherwise secured to bore 16. Window 24, when made of silver chloride, is coated with silver sulphide which absorbs visible and ultraviolet radiation; if the window is made of thallium bromide iodide no coating is required. The coating protects the silver chloride from actinic action. Housing 14 is preferably formed from silver or some other noble metal to inhibit reaction with the window. A radiation sensitive apparatus, generally indicated at 26, to be more fully described hereinafter, is cemented or otherwise secured to base 12 and includes a thermally sensitive flake 28 and a backing block 38. The ends of flake 23 are preferably gold coated and leads 32 and 34 are connected thereto and to larger leads 36 and 38 which in turn are connected to pins 40 and 42 supported in holes 44 and 46 in the base 12 by glass seals 48 and 50. These seals are connected to the metal base by solder seals 52 and 54 and pins 40 and 42 are preferably shaped and located to plug into a standard tube socket or the like. Accordingly the device may be connected into a circuit so that a biasing volt-age may be impressed across the flake 28 and signals from the flake amplified.

The structural details of the thermally sensitive apparatus 26 may be more readily comprehended from an examination of Figure 3 in which certain of the dimensions are greatly exaggerated for purposes of greater clarity. Thus flake 28 coated on its upper surface with a black lacquer absorbing layer 55 is joined to a thin plastic film 56 by a cement layer 58; the plastic film in turn is joined to a metal block 30 by the cement layer 60. This entire assembly is mounted on base 12 in any suitable manner. For example, if the backing block 30 is of copper or silver, and the base 12 is of a like metal, the backing block may be connected to the base by a solder seal 62. Electrodes 64 are attached to the flake to make electrical connection thereto. Accordingly heat may be transmitted from flake 28 through the cement layers 58 and 60 and plastic film 56 to the backing block 30 and from there to the base 12 for further dissipation. The superior operating characteristics of our improved thermistor bolometers are not only due to the coaction of the individual parts thereof, but also to their precise physical characteristics which will now be described in greater detail.

Thus, flake 28 is a resistor commonly known as a thermistor because of its high negative temperature co efficient and ability to change resistance value when infrared rays fall thereon. Preferably mixtures of oxides of manganese, nickel, and perhaps cobalt are used in making such flakes. Such mixtures are not conveniently expressed as parts by weight since the state of oxidation of the metals is not precisely known; they are preferably expressed in the number of atoms of the particular metal present per 100 atoms of the mixture. On this basis, preferred resistance materials comprise manganese to 20 nickel, or 52 manganese, 16 nickel and 32 cobalt.

When fast response rates are desired it is preferable that flake 28 be optically flat to minimize variations in thickness of the cement layer 58. As used herein the term optically flat means that the flake should pass between two plane parallel surfaces spaced apart no more than 5 microns greater than the flake thickness. For example, the standard 10 micron flake used in our thermistor construction, to be termed optically flat, should pass between two parallel surfaces 15 microns apart. The flakes are preferably generally rectangular in shape; typical flakes vary from 10 to 0.05 millimeters in length and from 10 to 0.05 millimeters in width.

Returning now to Figure 3 of the drawings, cement layers 58 and 60 are of minimum thickness, i.e. as thin as possible while still performing their adhesive functions. In practice, we have found that the average thickness of layer 58 should not be greater than 10 microns and preferably should be about 3 microns. Such layers in such range of thickness are hereinafter termed thin. Layer 60'is even thinner, since it is formed by coating the block 30 with a thin layer of cement, placing the plastic film 56 on this layer, and rolling it under pressure to squeeze out excess cement. Thus layer 60 is between 1 /2 and 3 microns thick. Cement layer 58 should be thin and as uniform as the variation in flake flatness will permit. The cement should not only intimately bond together flake 28, plastic film 56 and backing block 30, but should also offer minimum resistance to heat flow. Cement layer 58 must be an electrical insulator to avoid short circuiting the flake, although it is not desirable to depend upon it to insulate the flake from the backing block because of its extreme thinness. Plastic resins, especially the epoxy and phenolic resins, are preferred as a general class of materials to be used for the cements, since they provide strong but flexible bonds over wide temperature ranges. The thermal conductivity of the epoxy resins is about 5 10- calories/sec. C. cm. per centimeter of length, which is approximately the same as that of the plastic film 56, and is 2, that of the metals preferred for use in block 39 and base 12.

The plastic film 56 which insulates the electrically excited flake from the backing block should have a high dielectric strength in order to prevent voltage breakdown between the flake and the block. The film should also have very high electrical resistance to confine current flow entirely to flake 28, since extraneous currents cause noise. Further the film must be available in thin sheets, preferably less than 25 microns, in order that the low thermal conductivity of the film will not appreciably lower the speed of response of the thermally sensitive element. To prevent solvents used with the cements from changing the film thickness and perhaps perforating it, the plastic should not be affected by organic solvents. It is desirable that the film have high temperature stability to permit use of the element over wide ambient temperatures and it is also desirable that it be available in controlled thicknesses, i.e. thin films of varied uniform thicknesses. The availability of polyester films in controlled uniform thicknesses makes possible the tailoring of the time constant of the thermistor bolometer as previously explained. The material should be a thermosetting plastic so that high temperatures will not change its dimensions. We have found that polyester resin films satisfy most of these requirements and a preferred polyester is polyethylene terephthalate, a polymer formed by a condensation reaction between ethylene, glycol and terephthalic acid. This material has a dielectric strength of 4,000 volts/ mil. in thin sheets, and a resistivity of 10 ohm-cm. at room temperatures. In addition it is available in very thin sheets of uniform thickness and has excellent solvent resistance and temperature stability.

Returning to Figure 3, the backing block 30 should be a material with low thermal resistance, i.e. a good heat conductor. Since the thin plastic layer insulates the backing block from the flake, this backing block may be an electrical conductor. Accordingly, while any material having a high thermal conductivity is suitable, metals such as silver, copper and aluminum are particularly desirable in this application because of their exceptionally high thermal conductivity. It may be desirable to grind and polish the surface 30a of block 36 to insure that the cement layer 60 is as thin as possible.

In operation, infra-red energy falling on the window 24, is transmitted therethrough to the flake 28, whose temperature increases, causing a change in electric resistance. The current flowing through the flake because of the biasing voltage applied at the electrodes 64, changes and is detected by conventional electrical circuit means. in practice it has been found desirable to use alternating rather than direct current amplifiers to amplify the flake signal. In conventional apparatus used in conjunction with that shown herein the incoming radiation is chopped, i.e. periodically interrupted by a spoked rotating disc or the like so that the infra-red energy falling on the flake is in pulse form. Therefore,

it is desirable that the flake have a fast response rate in order to follow these pulses. By using a layer 56 of polyester resin of 6 microns thickness, an optically flat flake 28, and thin cement layers, we have succeeded in producing thermistor bolometers having time constants of between 1 and 2 milliseconds. This time constant is directly proportional to the thickness of layer 56 since almost the entire thermal resistance of the backing block is concentrated in it.

This time constant can be controlled by controlling the thickness of layer 56. For example, a plastic layer of 50 microns thickness will give a time constant of approximately 16 milliseconds, about 8 times that obtained with a 6 micron film, should a slower response rate be called for.

In addition to maintaining high response rates, we have found that this construction increases responsivity, and therefore the signal to noise ratio of the device. Referring to Figure 4, the responsivity of four thermistor bolometers having dilferent types of backing blocks all operating at a fixed proportion of permissible peak biasing voltage is there plotted as a function of the chopping frequency. It should be noted that the chopping frequency is plotted on a logarithmic rather than a linear scale, in order to include the entire frequency response in a reasonable space. Curve A represents the relative response of a glass-backed thermistor bolometer, curve B the response of a beryllium oxide-backed device and curves C and D the response of a device with a plasticcopper backing block. Curve C is obtained with a block having a plastic layer of 25 microns thickness While curve D is obtained with a plastic layer 6 microns thick It will be noted that the responsivity of the glass-backed bolometer decreases almost linearly with increasing frequency. The beryllium-oxide device does not decrease linearly, but drops ofl rather rapidly at low frequencies, is relatively flat in the midband range and at the higher frequencies again drops off. The flat area in the midband range is at a relatively low response. The response curves of the plastic-copper backed devices are similar in shape to the beryllium oxide curve, but the drop-off at low chopping frequencies is less marked and the response in the midband range is much bigger. As previously explained, this increase in responsivity is believed to be the result of the concentration of almost all of the thermal resistance of the backing block at a point immediately adjacent the flake. The flake thus tends to heat up very rapidly as a result of transient changes of energy impinging thereon, resulting in a greater resistance change and therefore larger output signals.

The relative speed of response of these devices is indicated by the frequency at which the response curve breaks, i.e. begins to drop 01f appreciably. Thus, a comparion of curves B and D indicates that although the copper-plastic backing block has greater responsivity, it has a response speed comparable to the beryllium oxide backing block, and both are much faster than either glass or quartz. Thus, by use of backing blocks made according to our invention, we have succeeded in making thermistor bolometers having increased responsivity, while at the same time maintaining a response rate as high as the fastest prior devices which are believed to be beryllium oxide backed.

Another important advantage of this construction is its high infra-red absorption factor. Energy falling on the flake which is not absorbed by the black coating or the flake itself passes through and is absorbed in part by cement layers 58 and 60 and by plastic film 56. The energy not absorbed by any of these layers is reflected from the surface 30a of the block which may be ground and polished and is returned through the cement and the plastic to the flake. Substantially all of the incident energy is thus absorbed by the flake, or materials in close thermal contact with it.

We have described herein an improved construction for thermistor bolometers which comprise a very thin layer of plastic film cemented to a metal block of high thermal conductivity. A flake of thermally sensitive resistor material is cemented to the film. If the flakes are optically flat and the cement layers and film are thin, the resulting thermally sensitive element Will have a response rate comparable to the best response rates obtainable with similar devices having backing blocks which are electrical insulators but thermal conductors. Further, thermally sensitive elements made according to this invention exhibit much greater signal output for a given amount of energy falling on them than prior constructions.

We have suggested that polyethylene terephthalate is particularly desirable for use as film 56 but it is to be understood that other plastic films having substantially similar characteristics may also give the improved performance described; therefore our invention is not to be limited to the particular materials mentioned. Further, although we have shown only a single sensitive element in the housing of Figures 1 and 2, it is obvious that a plurality of units, both shielded or unshielded may be included in such housing.

It will thus be seen that we have provided an improved construction for thermistor bolometers, and that the objects set forth above, among those made apparent from the preceding description, are efficiently attained.

Since certain changes may be made in the above constructions Without departing from the scope of the invention, it is intended that all matter contained in the above description, and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic inventions herein described and all statements of the invention, which, as a matter of language, might be said to fall therebetween.

We claim:

1. In a device responsive to infrared radiation, the

combination of an optically fiat flake of thermistor material adapted to be electrically excited, a metallic backing block of material having a high thermal conductivity adapted to serve as a thermal sink, a fihn of polyester resin of low thermal conductivity and of not more than 50 microns thickness, a first thin adhesive layer joining said polyester film to said backing block, and a second thin adhesive layer bonding said flake to said polyester film.

2. In a device responsive to infrared radiation, the combination of an optically fiat flake of thermistor material, a metallic backing block adapted to serve as a thermal sink, a filrn of polyester resin not more than 50 microns thick, an adhesive layer of not more than 5 microns thickness bonding said polyester film to said backing block, and a cement layer of not more than 10 microns thickness bonding said fiake to said polyester resin film.

3. In a device responsive to infrared radiation, the combination of an optically flat flake of thermally sensitive resistance material, a metallic backing block having a ground and polished surface adapted to serve as a thermal sink, a film of polyester resin of not more than 50 microns thickness having high electrical insulation resistance and low thermal conductivity, a first adhesive layer attaching said film to said ground and polished surface of said backing block, and a second thin adhesive layer bonding said flake to said film.

4. In a device responsive to infrared radiation, the combination of an optically flat thermistor element, a metallic backing block or" material having high thermal conductivity adapted to serve as a thermal sink and having a substantially flat surface, a film of polyethylene terephthalate of not more than 50 microns thickness, a cement layer joining such film to said substantially flat surface of said backing block, and means for securing said thermistor element to said polyethylene terephthalate layer.

5. The combination defined in claim 4 in which said metallic backing block is formed of material selected from the group consisting of silver, copper and aluminum.

6. in a device responsive to infrared radiation, the combination of an optically flat flake of thermally sensitive resistance material, a metallic backing block adapted to serve as a thermal sink and having a corresponding substantially fiat surface, a film of polyethylene terephthalate of not more than 50 microns thickness, said film being secured to said surface of said backing block by a cement layer of not more than 5 microns thickness, and an adhesive layer of not more than 10 microns thickness bonding said flake to said film, said backing block being formed of a material selected from the group consisting of silver, copper and aluminum.

References Cited in the tile of this patent UNITED STATES PATENTS 2,414,792 Becker Ian. 28, 1947 2,414,793 Becker et al Ian. 28, 1947 2,485,589 Gray et a1. Oct. 25, 1949 2,516,873 Havens et a1 Aug. 1, 1950 2,587,674 Aiken Mar. 4, 1952 2,633,521 Becker et a1 Mar. 31, 1953 2,742,550 Jenness Apr. 17, 1956 2,768,265 Ienness Oct. 23, 1956 FOREIGN PATENTS 744,176 France Jan. 21, 1933 OTHER REFERENCES OSRD 5991, Final Report on Development and Operating Characteristics of Thermistor Bolometers by J. A. Becker et al., Oct. 31, 1945, declassified May 6-10, 1946. Pages 10-14 relied on.

Proceedings of the Optical Society of America, vol. 36, No. 6, June 1946, pages 354-355.

Journal of the Optical Society of America, vol. 43, No. 1, January 1953, pages 15-21. 

